During the course of drilling an oil well, it is necessary at various intervals to lower casing or pipe into the hole and cement it in place. The cementing operation is accomplished by pumping a slurry down the pipe and up the annulus between the casing and the hole. Depending on the particular well and the assoicated circumstances, it is quite common to use one or more of a variety of additives to alleviate specific problems or achieve desired results. Thus, it is known and well documented in the art that additives can be used to either increase or decrease the density of the slurry; certain additives can be used to retard the thickening time, hence increasing the pumpability time; other additives can be used to accelerate the setting, and still others are used to expand the set cement during cure. Furthermore, various processes and methods of cementing have been perfected to achieve specific results, such as the process disclosed in U.S. Pat. No. 3,420,299, wherein an expanding agent is released at a time corresponding to the onset of the exotherm associated with the setting of cement. However, prior to our invention, the use of certain types of additives has been considered to be mutually exclusive in that their effects are opposite in nature and their simultaneous use would be counter productive. Thus, a cement accelerator additive is not ordinarily used concurrently with a cement retarder. However, in cementing deep hot wells, such as found in the Gulf Coast area, the need for both types of additives becomes apparent. Because of the extreme depths and the high temperatures, the cement slurry must be highly retarded in order to insure sufficient time to pump the slurry down the casing and back up the annulus.
As a consequence of being highly retarded, the time required for slurry to gel and set is prolonged. The longer the time that the cement slurry remains in an ungelled or unset state after placement the greater the odds are that gas channeling will take place necessitating a subsequent cement squeeze step. Furthermore, the presence of the retarder will also slow down the hardening process after setting. Since it is necessary for the mechanical strength of the cement to increase to a minimum value before work in the well hole can be resumed, the so-called "wait-on-cement" time is inherently extended, which, in practice, means prolonged down time and increased expense. It is also known that using a highly retarded slurry results in a lowering of the ultimate tensile strength of the cement. Thus, in a number of deep hot wells where highly retarded cement slurries were used, WOC times of up to 72 hours were required and unset or green cement has been found after several days following the cementing operation.
To alleviate these problems, we have developed a process of incorporating a cement accelerator into a highly retarded slurry without influencing pumpability of the slurry, yet, shortly after the placement of the slurry at the bottom of a well hole, the accelerator additive will essentially promote a flash set.